Mapping of ice storage processes on the Moon with time-dependent temperatures

Abstract:

Lunar cold traps are defined by extremely low sublimation rates, so that water ice is expected to accumulate in them. Physically, cold traps are defined by long-term integrated sublimation rates, but due to the strong dependence of the sublimation rate on temperature, they have heretofore been delineated by peak temperature. We developed a data processing technique that exploits information about the time variability of lunar surface temperatures measured by Diviner (the Lunar Radiometer Experiment on board Lunar Reconnaissance Orbiter).

Time-averaged sublimation rates are calculated for the south polar region of the Moon based on 11 years of Diviner surface temperature measurements. Data for each spatial pixel are binned according to subsolar and ecliptic longitude, and frequency-domain filtering of temperature time series is applied. The cold trap area defined by time-average sublimation rates is 17,000 km² from 80°S to the south pole. Subsurface ice stability is mapped based on solutions of the heat equation with Diviner surface temperatures as upper boundary condition. Even a thin layer of dust reduces the sublimation loss dramatically. Within two centimeters depth, the area with stable subsurface ice is twice that of surface cold traps. A third potential mechanism for ice storage, vapor pumping by temperature cycles, is also mapped, based on a model for the time variable population of adsorbed water molecules on the lunar surface.

Other ices ("super-volatiles") are trapped at lower temperatures than H₂O, close to the lowest temperatures measured within the lunar Permanently Shadowed Regions. Small spatially contiguous pockets of CO₂ ice stability are found in the craters Amundsen, Haworth, and Cabeus, totaling an area of 286 km². The LCROSS probe impacted one of those pockets and released, among other compounds, CO₂, serving as validation of the thermal stability calculations. The availability of carbon would be valuable for In-situ Resource Utilization (ISRU).